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Old 03-03-2012, 07:46 PM
gyula gyula is offline
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Join Date: Apr 2007
Posts: 820
Hi Jake,

Your biggest "problem" (may sound strange) is your closeness to the 10kW AM station... I mean the station makes too much field strength this is why almost anything behaves like an antenna, touching the coil makes big changes, etc.

First I would suggest using a coupling coil of 10-15 turns, placed to the bottom of your big air core coil, this coupling coil upper wire end would be connected to a 1 or 2m long wire (this latter serves as an antenna) and the lower end would go to Earth. Do not connect the big coil lower end to the coupling coil's earth point, ok?
Then you should find the capacitor needed for resonating your big coil to be a parallel LC circuit at 1620kHz. If you have now 300uH, then you would need 32.2pF to tune it to 1620kHz, the best would be to use a 40pF or 60pF trimmer capacitor. See link later for online LC calculator.
Then see this schematic here and I suggest making another 20-22 turn coil (not critical) onto your big air core coil, also to its bottom part, maybe over to the coupling coil mentioned above, this newer coil would serve as the L4 coil does in this schematic:
and you would need the diodes D1, D2, C3 and C4 of course, your crystal earpiece would connect to points A and B directly. (Do not connect this second coil to the earth point either.)

See this link where the schematics is referred to, it may be useful reading for you: High-Power Crystal Radio

I think your water tap earth is good enough but connect it ONLY to the antenna coupling coil I mentioned making first above, then you should not have 60Hz hum.

Another possibility is to use an AM ferrite antenna, like seen here:
Scotts Electronics Ferrite Loopstick Antenna Coil Crystal Radio Coils AM Coils but if you google you can find elsewhere too. Of course you do not have to make any antenna coupling coil for it, the Litze wire coil on the rod completes the antenna and if you choose the 680uh Litz wire coil with a tap at 470uh about 20 turns from end, then this 20 turn can serve as L4 for the diodes (you can separate the two wires which led out as a twisted wire to make the tap). This 680uH ferrite antenna needs 14pF capacitor across it.
I used this online calculator:
LC Resonance Calculator
(Notice the L value needs to be entered in nH, not in uH and f is in MHz.)

Normally it is better here to use higher L and lower C values but because of you want to receive a station at the very edge of the AM band the usual coils used for AM reception seems too high in value this is why so small pF value the tuning capacitor has (14pF or 32pF). You may wish to remove say 10-15 turns from your 60 turn air core coil to reduce its inductance and then increase the tuning cap higher or when using ferrite antenna the coil on the rod can be slided a bit off towards and / or beyond the rod's edge. The small cap values needed for 1620kHz resonance with such 300-680uH coils explain why you found good reception when you were holding the coil: your hand and body capacitance, your close presence to the coil supplied the 32pF tuning...

Hope this helps,

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